The findings could explain a long-running puzzle in climate change - why some regions in the northern hemisphere are strangely experiencing warmer winters than computer models have forecast.

Cities generate vast amounts of waste heat, from cars, buildings and power stations, which burn oil, gas and coal for transport, heating or airconditioning.

This phenomenon, known as the "urban heat island", has been known for years, but until now has mainly been thought to affect only city dwellers, especially in summer heatwaves.

But a team of scientists in the United States, using a computer model of the atmosphere, point to impacts that go much further than expected.

The high concentration of heat rises into jet-stream winds and widens their flow, transporting heat - as much as 1C - to places far away.

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The modelling sees autumn and winter warming across large parts of northern Canada and Alaska and in northern China.

The effect on global temperatures, though, is negligible, accounting for an average warming worldwide of just 0.01C.

"The world's most populated and energy-intensive metropolitan areas are along the east and west coasts of the North American and Eurasian continents, underneath the most prominent atmospheric circulation troughs," explained Ming Cai of Florida State University.